Method of protecting austenitic stainless steel subject to stress corrosion



United States Patent 3,378,359 METHOD OF PROTECTING AUSTENITIC STAINLESSSTEEL SUBJECT TO STRESS CORROSION v Kenneth R. Walston, Morristown,N.J., and Alistair S.

Couper, Chicago, Ill., assignors to Standard Oil Company, Chicago, 11].,a corporation of Indiana No Drawing. Continuation of application Ser.No. 378,445, June 26, 1964. This application Jan. 31, 1967, Ser. No.613,038

4 Claims. (Cl. 29-1962) ABSTRACT OF THE DISCLOSURE Austenitic stainlesssteel, having its exposed surface partially coated with aluminum, isused in a process of the type wherein the surface area of the steel isto be exposed to a stress-corrosive environment.

This is a continuation of our copending application Ser. No. 378,445,filed June 26, 1964.

This invention relates to improvements in the resistance of austeniticstainless steels to stress-corrosion cracking and more particularly to amethod of improving the resistance of austenitic stainless steels tostress-corrosion cracking by partially coating the stainless steel withaluminum and to the resultant stainless steel object.

Austenitic stainless steels are those with austenite and commonlycontain appreciable amounts of chromium and nickel. Usually the combinedchromium and nickel contentis greater than 24% with each element beingpresent in an amount equal to about 7% or more. Usually the carbon isvery low. One of the common austenitic stainless steels is the 18-8 typecontaining about 18% chromium and 8% nickel.

The austenitic stainless steels are known to have the problem ofstress-corrosion cracking. This problem is discussed in such referencesas Samans, Carl H.: Engineering Metals and Their Alloys, third printing,New York, The MacMillan Company, 1952, p. 621 and Chemical Processing,Costly Stress-Corrosion Cracking Curbed, Oct. 21, 1963, issue. Generallythe problem has been encountered when the stainless steel has been usedin a chloride or sulfide environment.

Stress-corrosion cracking in austenitic stainless steels is particularlytroublesome since the damage is caused by .cracks which in manyinstances are quite deep and significantly reduce the effective life ofthe stainless steel compared to the slower effects of ordinarycorrosion. Because of the problems associated with stress-corrosioncracking in austenitic stainless steels, it is an object of thisinvention to improve their resistance to cracking. Other objects willbecome apparent from the detailed description below.

In our elforts to improve the resistance of austenitic stainless steelsto stress-corrosion cracking, we have discovered that such resistance isremarkably improved when the stainless steel is partially coated withaluminum. It is not necessary that the aluminum cover the entire surfaceof the austenitic stainless steel to provide the improved resistance.However, it is advantageous to have the aluminum substantiallydistributed over the surface of the stainless steel to increase theelfectiveness of the protection.

Briefly, the invention is directed to a method of improving theresistance of austenitic stainless steel to stresscorrosion cracking,which method comprises partially coating stainless steel with aluminum.Useful forms of the coated stainless steel are vessels, exchangers,pipes, and the like.

In the method of this invention, the austenitic stainless steel ispartially coated with aluminum. The coating process may be carried outby spraying, welding, dipping, or the like. Advantageously, the partialcoating should be substantially distributed over the surface of thestainless steeL'A stainless steel which is particularly of interest forthis protection contains about 18% chromium and about 8% nickel.

Advantageously, the coating process may include a subsequent heattreatment to improve the bonding of the aluminum to the stainless steel.The heat treatment is commonly carried out at temperatures up to about1300 F. and in an inert atmosphere, although the latter is notessential. At 1300 F., the time for the treatment is commonly about onehour.

The resultant austenitic stainless steel with its partial coating ofaluminum exhibits improved resistance to stress-corrosion cracking. Theprotected stainless steel may be utilized in numerous ways. One ofparticular interest is as a stainless steel vessel.

The particular benefits of the aluminum protection is of primaryinterest when the stainless steel is subjected to the environment ofchloride or sulfide, since these environments are usually associatedwith stress-corrosion cracking.

The following examples are some embodiments illustrating the invention.It is to be understood that these are for illustrative purposes only anddo not purport to be wholly definitive with respect to conditions orscope.

Example I A test was carried out in which three 18-8 stainless steelspecimens in the form of horseshoes were subjected to boiling magnesiumchloride. The first specimen was unprotected, the second specimen wasattached to two pieces of aluminum, and the third specimen was partiallycoated with aluminum. Each specimen was prepared by bending a stainlesssteel bar into a horseshoe shape with a bolt then being inserted throughthe two ends of the horseshoe to prevent any outward movement of theends. Each of the aluminum pieces for the second specimen was placed onthe bolt adjacent to each end of the horseshoe. The third specimen wasprepared by spraying aluminum on the stainless steel bar and thenbending the bar into the horseshoe shape. This bending caused thealuminum layer to separate at the location of the maximum bend andexpose the stainless steel.

The magnesium chloride solution was prepared from approximately 350grams of .MgCl 6H O and 40 milliliters H O. The test was carried out ina reflux flask and at a temperature of approximately 307 F. (153 0.).

Each specimen was individually tested by placing it in the boilingsolution, and the test was carried out until failure of the specimenoccurred. The failure point occurred when the cracks in the stainlesssteel specimens were sufficiently deep to permit the collapse of thehorseshoe shape when its two ends were manually moved inwardly.

In the test, it was observed that the unprotected specimen failed inabout 4 hours, the specimen attached to the aluminum pieces failed inabout 17 days, and the specimen partially coated with aluminum failed inabout 38 days. These results demonstrate that the partial coating ofaluminum more than doubled the life of the stainless steel specimen inthe chloride environment.

Example II A second test was carried out in which stainless steelhorseshoes, of the type-described in Example I, were subjected topolythionic acid at approximately F. The acid was prepared by bubbling H8 through an aqueous solution saturated with S0 until the solutionbecame saturated with respect to H S.

In the test the unprotected specimen failed after about an hour, thespecimen attached to the aluminum pieces failed in about 1 hour also,and the specimen partially coated with aluminum failed in about 17hours. The above results demonstrate that the partial coating ofaluminum increased the life of the stainless steel in the sulfideenvironment by a multifold factor.

While the invention in its various aspects has been described withreference to particular embodiments thereof, it is apparent that theseare by way of illustration only. Accordingly, it will be understood thatmodifications and variations thereof will be apparent to those skilledin the art, and it is thus intended to embrace all such modificationsand embodiments as fall within the broad scope of the appended claims.

We claim:

1. In a method of utilizing austenitic stainless steel subjected tostress corrosion, the improvement providing the exposed surface with apartial aluminum coating in discontinuous portions distributed over theexposed surface of the steel.

2. The method of claim 1 wherein said stainless steel contains about 18%chromium and about 8% nickel.

3. The method of claim 1 wherein said environment is a chlorideenvironment.

4. The method of claim 1 wherein said environment is a sulfideenvironment.

References Cited UNITED STATES PATENTS 2,171,040 8/1939 Merritt 29l96.22,908,073 10/1959 Dulin 29197 3,167,403 1/1965 Smith 29196.2 3,173,2023/1965 Farber 29196.2 3,210,840 10/1965 Ulam 29-196.2

2g HYLAND BIZOT, Primary Examiner.

